1. Field of the Invention
This invention relates to an ignition device of header assemblies used for example in an inflator system employed in automobile safety equipment, and the like, and methods of manufacture thereof.
2. Description of the Prior Art
Various safety devices, such as air bags and seat belt pretensioners, have been implemented to protect automobile drivers and passengers. In the event of an accident the life-saving rate of these safety devices is high, and the proportion of vehicles to which they are fitted is increasing.
However, it is essential that when an accident has not occurred, automobile safety devices such as air bags remain in a state such that they will function correctly for the long period of time that elapses until the vehicle is disposed of. Accordingly, high reliability is desirable. It is also essential that such safety devices operate rapidly and correctly if an accident does occur.
For reasons such as these, explosive materials are widely used to activate automobile safety devices, and the like. Furthermore, electrical header assemblies are often used to ignite the explosive material.
Taking the air bag as an example of an automobile safety device, conventional header assemblies will now be explained with reference to FIG. 7. The item labelled 71 is a cylindrical case sealed at one end, and an eyelet 72 has been fitted into the lower half of the inside of the cylindrical case 71. Ignitor charge 73 has been loaded into the space above the eyelet 72, and a pair of stem electrodes 74a and 74b are provided that pierce through the eyelet. Both ends of the bridge element 75 are electrically connected to top surface of the stem electrodes 74a and 74b by welding. The bridge element 75 is in close contact with the ignitor charge 73. Insulating material 76 is placed around the stem electrodes 74a and 74b, so that the stem electrodes 74a and 74b do not come into contact with each other.
In the above construction, if a vehicle fitted with the air bag is involved in a collision, electric current flows in the stem electrodes 74a and 74b due to a collision detection signal. This current flows through the bridge element 75, and the bridge element 75 heats up and fuses. The ignitor charge 73 is ignited by the thermal energy released, gas-producing material caused to react, and the safety device caused to operate.
Prior art header assemblies have used resistance wire as the bridge element, as in U.S. Pat. No. 3,971,320. Furthermore, by means of resistance welding, and the like, this resistance wire has been connected between stem electrodes exposed on the stem surface. The dimensions of the resistance wire have been selected in accordance with the required ignition properties and the power supply used to cause current to flow between the stem electrodes. For example, wire with a diameter of several tens of micro-metres, and a length of several millimetres has been used.
However, variations in welding strength occur when the wire bridge element and the stem electrodes are welded together. In addition, the stress involved when bridge element is brought into close contact with the ignitor charge 73 varies. As a result, with prior art header assemblies factors which prevent the provision of uniform operating properties occur within the manufacturing processes, and the like, and this decreases reliability.
It has been proposed, as in U.S. Pat. No. 5,404,263, that the occurrence of stress be prevented by making these stem surfaces flush. However, the bridge element is welded directly to the electrodes.
In order to stabilise the operating properties of the bridge element, it is important to maintain a constant distance between the welded parts at both ends of the bridge element welded to the stem electrodes, and this requires high-precision control of the welded parts of the bridge element. However, it is very difficult to control the precision of welding points because the bridge wire is both thin and short. Furthermore, the control needed for precision would result in a reduction in the ease of mass production.
In addition, a construction has been proposed, as in Japanese Patent Laid-open 64-75896, wherein a thin deposited film is formed as the heating body, by sputtering, vapour deposition, or the like, onto a ceramic substrate. However, this has disadvantages such as the fact that junction with the stem electrodes is achieved by soldering, electrode caulking, and the like, and because the thin film is formed by film deposition, it is impossible to produce a heating body with a precise resistance value.
This invention has as its object the provision of a highly reliable easily mass produced ignition device and a method of manufacture thereof.
This invention has as its object the provision of an ignition device, ignition device provided with a bridge element positioned on top of a stem, and heating carried out by passing an electric current through this bridge element from stem electrodes provided in the stem, in which the device comprising:
a stem comprising at least two mutually separated stem electrodes and an insulating element provided between these stem electrodes to insulate and maintain the stem electrodes and to form a stem surface in conjunction with the ends of the stem electrodes; and
a heating element comprising a bridge element provided with a heating bridge wire and heating electrodes positioned at the both ends of the heating bridge wire, and a flexible insulating sheet positioned on top of the stem surface, on which the bridge element is supported, and in which openings or cut-out areas are formed as connecting guides corresponding to the position of the heating electrodes, wherein the connecting guides of the flexible insulating sheet are aligning with the ends of these stem electrodes, and the heating electrodes of the bridge element are electrically connected to the ends of the stem electrodes.
In addition, this invention has as its object the provision of a method for manufacturing an ignition device used in an inflator system, comprising a step in which a metal resistor sheet is joined to a flexible insulating sheet in which a plurality of openings spaced to correspond to the spacing of the stem electrodes have been formed in advance, a step in which the metal resistor sheet is etched to form a plurality of units each comprising a thin heating bridge wire and, connected at both ends of the heating bridge wire, heating electrodes wider than the heating bridge wire, and in which the heating electrodes are positioned above the said openings, a step in which the heating electrodes are welded to the stem electrodes, and a step in which the insulating sheet is cut so that single units comprising the heating bridge wire and the heating electrodes attached to both sides of this heating bridge wire remain on the insulating sheet.